It is desirable to be able to detect a fuel leak from an aircraft fuel tank. There are several different possible causes for fuel leaks in an aircraft including foreign objects impacting against the fuel tank or associated system, and development of faults in the fuel tank and associated system resulting from fatigue and/or wear and tear. More specifically, and by way of example, in an aircraft potential causes of fuel tank leaks include improper maintenance, improper manufacture, natural component ageing, uncontained engine rotor failure, battle damage, damage from impact of other foreign objects (for example, debris on the runway during take off or landing), system malfunctions or errors (for example, during to air to air refuelling, jettisoning fuel, de-fuelling or unintentional over-pressurising of the fuel tanks) and general component malfunction (for example, including the malfunction of components not directly associated with the fuel tank system which could subsequently cause damage to the fuel tank system). Fuel may of course leak from the fuel tank itself or any part of the aircraft in which fuel is present. By way of example, fuel may leak directly from a hole in the tank structure, from a hole in a fuel pipe or from a faulty seal between fuel pipes, or may leak from the surge tanks (the surge tanks accommodating overflow of fuel in the rest of the fuel system but only having a limited capacity which, if exceeded, leads to fuel being expelled out of the aircraft to atmosphere).
One method of detecting a fuel leak is to compare the amount of fuel used by the aircraft with the decrease in the quantity of fuel stored in the fuel tank. If the decrease in fuel stored is greater than quantity of fuel used, it may be concluded that there is a fuel leak. There are however difficulties in accurately and promptly detecting fuel leaks in an aircraft. For example, it is difficult to obtain an accurate measure of the amount of fuel in the fuel tanks because the fuel in the tank moves within the fuel tank as a result of movement and vibration of the aircraft so that measurements of the amount of fuel in the fuel tank are subject to error. Of course measurements relating to other parameters, from which the amount of fuel being used can be ascertained are also subject to a certain level of error.
A fuel leak detection system currently in use on an aircraft will now be described. The system decides whether or not there may be a fuel leak by comparing the decrease in the quantity of fuel stored with the quantity of fuel which has flowed from the tank to the engine over the same period. The difference in the two quantities can be expressed in algebraic form as follows:fuel discrepancy=FQI−∫fuel rate dt−TOFOBwhere    FQI=current fuel quantity indication,    fuel rate=measure of rate of fuel flow from a flow meter(s) in the fuel pipeline (a negative value, since fuel leaves the feed fuel tanks and enters the engine), and    TOFOB=take-off fuel on board.
A leak parameter is obtained by passing the discrepancy signal through an n-point moving average filter where typically n is greater than 50. So thatleak parameter=filter {FQI−∫fuel rate dt−TOFOB}  (1)
The function of the filter is to reduce the effect of random measurement errors usually referred to as noise which primarily affect the FQI measurement but are also present to a lesser extent in the fuel rate measurement.
In an ideal situation the leak parameter would only deviate from zero if a leak was present. The imperfect nature of the filter and the presence of systematic or offset measurement errors, however, mean that the leak parameter must exceed a pre-specified threshold to be indicative of a leak with the appropriate degree of confidence.
It has been found however that the currently used method suffers from a number of disadvantages as will now be explained. In order to avoid or reduce the number of false alarms the threshold, which the fuel leak parameter must exceed before the system deems that there is a fuel leak, is set relatively high. For example, a typical fuel tank gauge for making FQI measurements may only be required to provide FQI measurements such that the error in the measurements, whilst the fuel tank is greater than 20% full of fuel, is ±2%. The capacity of the fuel tanks might be of the order of about 55 tonnes (the fuel used between take-off and landing or refuelling typically being up to about 50 tonnes taking account of a minimum fuel reserve of about 5 tonnes). Thus, when the fuel tank is full the fuel leak system assumes an error in the FQI measurement of about 4% of the fuel on board, equating to about 2.2 tonnes of fuel. For example, consider the case where the amount of fuel in the tank is exactly 50 tonnes. The FQI measurement could be between 49 and 51 tonnes. The leak measurement system must however assume an error of ±2%, so that a FQI measurement of 49 tonnes might result from an actual fuel quantity of 50 tonnes or 48.04 tonnes, the latter value being about 4% different from the real quantity. The threshold is therefore set to be above 2.2 tonnes. By providing a relatively high threshold that the leak parameter has to exceed before a potential leak is detected, a significant quantity of fuel may be lost before detection occurs.
The present invention thus seeks to provide an improved method of leak detection and/or to provide a method of detecting leaks that mitigates one or more of the above-mentioned disadvantages associated with the above-described method currently in use.